1. Field of the Invention
This present invention is directed to drilling operations, systems for controlling the orientation of a drill bit during drilling, and, in certain particular aspects, to controlling bit face orientation during drilling.
2. Description of Related Art
The prior art discloses a wide variety of drilling systems, apparatuses, and methods which use a rotary drive or top drive drilling system with a motor that rotates a drive shaft which in turn rotates a drillstring; including, but not limited to, the disclosures in U.S. Pat. Nos. 6,944,547; 6,918,453; 6,802,378; 6,050,348; 5,465,799; 4,995,465; 4,854,397; and 3,658,138, all incorporated fully herein for all purposes. One of the challenges for directional drilling is ensuring the directional motor is oriented properly for the desired change in drilling direction. This requires the top drive to move the string in order to move to specific positions rather than simply blindly rotating the shaft.
Certain current top drive control interfaces and software allow a driller to perform bit-face orientation movements with a top drive, but often these systems are inaccurate. In one method, the top drive is rotated by applying a speed command (throttle) and a torque limit after selecting a direction. With variable frequency drive top drives, the operator can watch the top drive shaft while slowly opening the throttle and can use the throttle control to stop the shaft when it is in the desired position. This is using the driller as a closed-loop position control portion of the operation, which can be undesirable. For HMI-based human-machine interface top drives, the situation can be worse since the driller must key in a throttle on a touch screen, watch the movement of the drive, then quickly look back at the screen and hit “zero throttle” in order to stop the shaft. This can lead to errors.
In directional drilling, in which target formations may be spaced laterally thousands of feet from a well's surface location requiring penetration to depth and also laterally through soil, rock, and formations, bit direction is determined by the azimuth or face angle of the drilling bit. In certain prior systems, face angle information is measured downhole by a steering tool and, typically, conveyed from the steering tool to the surface using relatively low bandwidth mud pulse signaling. A driller maintains a desired face angle by applying torque or drillstring angle corrections to a drillstring, but because of the latency or delay in receiving face angle information, the driller often over- or under-corrects. The over- or under-correction can result in substantial back and forth wandering of the drill bit, which increases the distance that must be drilled in order to reach the target formation. Back and forth wandering can also increase the risk of stuck pipe and make the running and setting of casing more difficult.
In directional drilling, especially in long reach, high angle, or horizontal drilling, long bit runs, smooth and properly controlled well paths, and minimal course corrections are desirable. In actual drilling, many downhole trajectory control devices are used to deflect the drilling trajectory whenever necessary. These include downhole bent housings of the downhole motor, bent subs or whipstocks, and other active or adjustable devices such as adjustable stabilizers. To properly execute the trajectory deflection, it is very important to set the tool face accurately.
One prior method of setting the tool face angle relies on measuring the tool face angle at the location where downhole survey sensors are located in a BHA (bottomhole assembly). However, due to the interference fit caused by such downhole deflection devices, significant contact forces are generated by such devices at the contact points (i.e., the bent knee and the intervening stabilizers). These restraining torques prevent the bent knee from turning when the surface torque is applied. Therefore, the “apparent tool face” at the sensor location can very often differ significantly from the true tool face angle at the bent knee.
One prior method of downhole tool face setting is to infer a tool face orientation at the axial location where the survey sensors are located through survey measurements. The effect of the “restraining torque” at the bent knee and any other intervening contact locations (such as the upper stabilizer of the downhole motor) may not be accounted for. As a result, not only is accuracy affected, but also the azimuth accuracy of the directional survey, since the survey data are influenced by the deformation of the downhole assembly. Often the azimuth accuracy in an MWD survey, particularly near the horizontal section, can be very poor. Errors of over two degrees in azimuth from such surveys are fairly common. The uncertainty of the well trajectory, due to such azimuthal error, will either lead to strayed drilling or to a crooked horizontal well path. This can limit the maximum drillable horizontal extent of the well.
In rotating a drillstring to rotate a bit to a desired orientation, it is desirable to achieve a new bit face orientation as quickly and accurately as possible, but without fast, jerky movements which may result in overshooting or undershooting a desired bit location.